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IEEE 802.11 - Wireless Local Area Networks

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Infrastructure Mode (1)

transport layer

network

layer

LLC

(layer 2b)

Access Point (AP): Any entity that has STA functionality and provides access to the DS via the wireless medium (WM) for associated stations => AP implements both the 802.11 MAC and the DS MAC protocols.

Communication Technology Laboratory

Wireless Communication Group

Wireless Networks, 802.11

Access Point

(AP) A

AP B

Infrastructure Mode (2)

Several connected BSSs may form (together with integrated LANs) an Extended Service Set (ESS)

The architectural component used to interconnect BSSs is the distribution system (DS).

TSIDs are themselves based upon Transmission Specification (TSPEC) parameters like data rate, burst size, and service interval.

This parameterized QoS mechanism is arguably more complex than prioritized QoS.

E.g., HCCA requires STAs to know what they'll want to send in advance.

However, in WLANs used primarily for voice or video streams, HCCA with well-tuned QoS parameters can enable more efficient channel utilization by eliminating "wasted" backoff time.

Controlled Access Phase (CAP) Generation

HC shall sense the WM.

If WM is determined idle for one PIFS period, HC transmits first frame of any permitted frame exchange sequence.

Duration value is set to cover the CFP or the TXOP.

First permitted frame in a CFP after a TBTT is the Beacon frame.

(controlled access phase)

802.11e

Access Point

SIFS

SIFS

SIFS

Piggybacking

(Reduces Poll and Ack Overheads)

Wireless Station

PIFS

PIFS

HCCA TXOP

UPLINK TXOP

DOWNLINK TXOP

Polled TXOP limit

Downlink TXOP limit

Beacon

Poll + Data

Ack

Data

Busy

Data

Ack

t

SIFS

SIFS

SIFS

Ack

Data

Data + Ack

t

802.11e

WLAN IEEE 802.11e

Introduction

MAC Architecture

DCF

HCF: EDCA, HCCA

Main Differences from legacy IEEE 802.11

Traffic Classification

TXOP

Polling during CP

Admission Control

Block Acknowledgment

No Acknowledgment

DLS operation

802.11e

Admission Control

Important for the provision of guaranteed QoS parameters

Goal: Limit amount of traffic admitted into a service class so that QoS of existing flows will not degrade, while the medium resources can be maximally utilized

An IEEE 802.11 network may use admission control to administer policy or regulate the available bandwidth resources.

Admission control is also required when a QSTA desires guarantee on the amount of time that it can access the channel.

HC administers admission control in the network.

Admission control, in general, depends on vendors’ implementation of the scheduler, available channel capacity, link conditions, retransmission limits, and the scheduling requirements of a given stream.

802.11e

Block Acknowledgment

The Block Ack mechanism improves channel efficiency by aggregating several acknowledgments into one frame.

There are two types of Block Ack mechanisms: immediate and delayed.

The Block Ack mechanism is initialized by an exchange of ADDBA (add Block Acknowledgment) Request/Response frames.

The number of frames in the block is limited, and the amount of state that is to be kept by the recipient is bounded.

Acknowledgments of frames belonging to the same TID (Traffic Identifier assigned by higher layers), but transmitted during multiple TXOPs, may be combined into a single BlockAck frame.

802.11e

No Acknowledgment

ACK does not need to be used in a QBSS in case of time-critical services when a retransmission is not reasonable.

There is no MAC-level recovery, and the reliability of this traffic is reduced, due to the increased probability of lost frames from interference, collisions, or time-varying channel parameters.

A protective mechanism (such as transmitting using HCCA, RTS/CTS, should be used to reduce the probability of other STAs transmitting during the TXOP.

802.11e

DLS (direct-link setup) Operation (1)

DCF: In general, STAs are not allowed to transmit frames directly to other STAs in a BSS (exception: IBSS, i.e. Ad-hoc network)

They should always rely on the AP for the delivery of the frames

However, STAs with QoS facility (i.e., QSTAs) may transmit frames directly to another QSTA by setting up such data transfer using DLS.

Need for this protocol: motivated by the fact that the intended recipient may be in PS mode, in which case it can be awakened only by the QAP.

Second feature of DLS: exchange of rate set and other information between the sender and the receiver

DLS prohibits STAs going into PS mode for the duration of the direct stream as long as there is an active DLS between the two STAs.

DLS does not apply in a QIBSS, where frames are always sent directly from one STA to another.

The completed version 1.0 WiGig specification was announced in December 2009.

In May 2010, WiGig announced the publication of its specification, the opening of its Adopter Program, and the liaison agreement with the Wi-Fi Alliance to cooperate on the expansion of Wi-Fi technologies.

In June 2011, WiGig announced the release of its certification-ready version 1.1 specification.

WiGig specification will allow devices to communicate at multi-gigabit speeds.